1. Technical Field
This invention relates to metal-ceramic composite materials, and more particularly to metal powder particles which, after being coated with a thin layer of ceramic, glass, or glass-ceramic bonding media insulation are densified by application of heat, with or without pressure.
2. Description of the Related Art
The cores of motors, generators, and transformers are generally comprised of a large number of thin metal laminations that are separated from one another by a layer of insulating material. In the highest performance magnetic cores the metal laminations are comprised of one of two iron-cobalt alloys (Fe-49Co-2V or Fe-27Co-0.6Cr). The interlaminar insulation, which may consist of an oxide layer on the metal plus an organic adhesive layer between laminations, is necessary to insure high electrical efficiency in the magnetic core. The most demanding applications for these core assemblies are those used in airborne power generators. Airborne power generation requires compact, high-output equipment and thus a lamination material with the highest saturation induction and lowest hysteresis losses, i.e., an iron-cobalt alloy. The high rotational speeds in these devices, on the order of 12,000 rpm, imposes significant mechanical stresses on the rotor material as well as the adhesive that bonds the laminates. In fact, the yield strength of the magnetic rotor material may be the decisive factor in alloy selection for this application, and it is highly desirable that the strength of the adhesive bond be comparable with that of the magnetic material.
There are presently under development two new demanding applications for magnetic materials--compact, very high speed electrical generators, and high-temperature magnetic bearings. The proposed generators spin at speeds on the order of 100,000 rpm, resulting in high stresses on the foil laminates and the adhesives joining them.
The high-temperature magnetic bearings are being considered for future gas turbine engines. Magnetic bearings could increase the reliability and reduce the weight of these engines by eliminating the lubrication system. They could also increase the DN (diameter of the bearing times rpm) limit on engine speed, and allow active vibration cancellation systems to be use--resulting in a more efficient, "more electric" engine. The magnetic bearing is similar to an electric motor. It has a laminated rotor and stator, likely made of an iron-cobalt alloy. Wound around the stator are a series of electrical wire coils that form a series of electric magnets around the circumference. The magnets exert a force on the rotor. A probe senses the position of the rotor, and a feedback controller keeps it in the center of the cavity. For gas turbine applications, it is desirable that the magnetic bearings be capable of operating at temperatures on the order of 650.degree. C.
The strength of magnetic rotor assemblies can be enhanced by the addition of metal pins or stakes that are inserted into holes punched in the laminations. However, there is a penalty in electrical efficiency for the use of such devices since the lamination factor (solidity of the core) is reduced when the magnetic lamination material is replaced by a non-magnetic material.
Magnetic core material can also be made from metal powder instead of foil. This approach has several advantages over the more traditional foil lamination technique: (1) the material will be more isotropic in magnetic and mechanical properties than a laminated product, (2) the size of the core is not limited, as is a laminated core in "pancake" geometry, by the available width of magnetic alloy foil, and (3) the core can be fabricated from an alloy (such as Fe-6Si) that is too brittle to roll into foil. However, for AC applications the metal particles must still be electrically isolated from one another in order to minimize eddy current losses.
At least one such powder metallurgy product is presently commercially available from the Hoeganaes Corporation of Riverton, N.J. See also U.S. Pat. Nos. 5,063,011, 5,198,137, 5,268,140 and 5,300,317. This material is comprised of thermoplastic-coated iron particles that are formed into a structure through application of very high pressures at warm temperatures, such as 50 tons/in.sup.2 and 260.degree. C., respectively. The thermoplastic coating serves as the electrical insulation between particles (as required for AC applications) as well as the bonding agent. The surface of the iron particles may also be pretreated, such as with a phosphate coating, as an added insulating material.
However, organic materials (whether in the form of an organic adhesive used to bond metal foil laminations, or a polymer coating used to insulate and bond metal powder particles) lose much of their strength at relatively modest temperatures. For example, according to the chapter "Adhesives Selection" by John Williams in the ASM Engineered Materials Handbook, Volume 1, Composites, p. 684, (1987), the maximum use temperatures for organic adhesives range from only 82.degree. C. for epoxies to 260.degree. C. for some polyimides. Thus, the strength of organic-bonded magnetic structures can be expected to be severely degraded by temperatures as low as 100.degree. C. to 200.degree. C.
Thus, there is a need for a method to strongly bond together magnetic alloy particles to form the cores of high performance electromagnetic equipment. The method would replace conventional foil laminate structures comprised of metal foils bonded by organic adhesives, as well as the newer powder metallurgy products in which a thermoplastic (or other organic material) is used to electrically isolate and bond together magnetic alloy particles.